Before plummeting beneath the clouds forever, the European Space Agency(ESA)’s Venus Express unearthed a few more secrets about our sister planet. Its final months in low orbit revealed the planet to be much colder and far less dense than expected.

The craft was launched over ten years ago on the 9th November, 2005. It began to orbit Venus in 2006 and was able to maintain its orbit for 8 years, despite initial estimates placing it at 500 days. In 2014, Venus Express finally ran out of fuel and fell into low orbit, losing contact with Earth in 2014.

In its last few months in low orbit, the craft experienced measurable drag from Venus’ atmosphere. Using onboard accelerometers, Venus Express was able to measure its deceleration as its orbit decayed. This is a process known as aerobraking, and it has taught us a lot about the nature of Venus’ atmosphere.

“None of Venus Express’ instruments were actually designed to make such in-situ atmosphere observations,’ says Ingo Müller-Wodarg of Imperial College London. ‘We only realised in 2006 – after launch! – that we could use the Venus Express spacecraft as a whole to do more science.”

And indeed, in its final months afloat, the Venus Express gathered some fantastic data about Venus. Previous models of Venus’ polar atmosphere was based on data gathered by NASA’s Pioneer Venus probe in 1978. However, Venus’ specific polar atmosphere was never studied on site.

Müller-Wodarg and colleagues have discovered that the polar atmosphere of Venus is up to 70 degrees colder than expected. Its average temperature measures at -157°C (or 114 Kelvin). The atmosphere was also discovered to be far less dense than expected. 22% less dense at 130km altitude, and 40% less at 140km. Venus Express also discovered atmospheric waves at these altitudes. Such waves are thought to be highly important in shaping a planetary atmosphere.

“This is in-line with our temperature findings, and shows that the existing model paints an overly simplistic picture of Venus’ upper atmosphere,” says Müller-Wodarg. “These lower densities could be at least partly due to Venus’ pilar vortices, which are strong wind systems sitting near the planet’s poles. Atmosphere winds may be making the density structure both more complicated and more interesting!”

In other words, Venus is cold and complicated, truly the most Tsundere of planets. Venus Express is Europe's first successful voyage to Venus. It is not only interesting in its own right, but has great implications for the ESA’s ExoMars mission, which has recently begun. One can only wonder at the secrets we'll uncover about our other, redder neighbour in the years to come.